1. Field of the Invention
The present invention relates to a Broadband Wireless Access (BWA) system. More particularly, the present invention relates to a scheduling method for a BWA system that is capable of improving network throughput and power conservation performance of subscriber stations.
2. Description of the Related Art
Recently, it has been considered desirable to integrate commercially available wireless networks such as Local Area Network (LAN) and Metropolitan Area Network (MAN) into a universal access platform, also known as the fourth generation (4G) communication system for securing mobility and Quality of Service (QoS) while maintaining relatively high data rates.
The Institute of Electrical and Electronics Engineers (IEEE) 802.16a and 802.16e Wireless MAN standards specify wireless metropolitan area networks adopting the Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) for supporting broadband wireless transmission on physical channels.
Using the OFDM/OFDMA schemes in which signals are transmitted through a plurality of subcarriers, the IEEE 802.16a and 802.16e systems can achieve high speed data transmission. That is, the IEEE 802.16a and 802.16e systems are BWA systems based on the OFDM/OFDMA schemes.
FIG. 1 is a schematic diagram illustrating a conventional BWA communication system.
Referring to FIG. 1, a BWA system has a multi-cell structure, i.e. cells 100 and 150 defined by individual base stations 110 and 140 that provide access services to subscriber stations 111, 113, 130, 151 and 153. The base stations 110 and 140 provide access to the system for the subscriber stations 111, 113, 130, 151 and 153 on the basis of the OFDM/OFDM schemes.
In the OFDMA scheme, a subchannel includes a group of subcarriers constituting an OFDM symbol, and a frame includes multiple OFDM symbols.
FIG. 2 is a diagram illustrating a frame structure for use in a conventional BWA system. More particularly, FIG. 2 shows an OFDM frame structure of a BWA system operating in a Time Division Duplexing (TDD) mode.
As shown in FIG. 2, each frame includes a plurality of OFDMA symbols indicated by horizontal OFDMA symbol numbers and a plurality of subchannels indicated by vertical subchannel logical numbers. Also, each frame includes Downlink (DL) and Uplink (UL) subframes separated by Transmit/Receive and Receive/Transmit Transition (TTG and RTG, respectively) gaps.
Each DL subframe starts with a preamble followed by a Frame Control Header (FCH) and a DL-MAP that are commonly broadcast to all the subscriber stations.
The preamble contains information for acquiring the synchronization between a base station and subscriber stations, i.e., preamble sequence.
That is, the preamble is required for synchronization of data transmitted by the base station, whereby a modem of the subscriber station extracts synchronization information from the preamble in various manners.
The FCH, which includes two subchannels, contains basic information on the subchannel, ranging and modulation scheme. By analyzing the information carried by the FCH, each subscriber station can recognize a burst profile and length of the DL-MAP immediately following the FCH, and a frequency reuse factor of the base station typically set to 1 or 3.
The DL-MAP delivers a DL-MAP message carrying various information required for extracting data and providing services to the subscriber stations. The data can be extracted from the DL frame on the basis of the information contained in the DL-MAP message.
The downlink subframe includes multiple zones. Within each zone, transmission resources are allocated in bursts. For example, the zone composed of the (k+3)th to (k+15)th OFDM symbols is divided into 6 data bursts labeled DL burst #1 to DL burst #6. The data contained in each burst is extracted on the basis of the analysis of the DL-MAP.
The UL subframe includes one or more zones. Within each zone, transmission resources are allocated in respective bursts. The zone composed of (k+17)th to (k+26)th OFDM symbols is divided into 5 data bursts labeled UL burst #1 to UL burst #5 and a ranging region. The ranging region is composed of ranging subchannels for ranging and bandwidth request purposes. The uplink data are extracted from the UL bursts on the basis of the information acquired from the UL-MAP carried by the DL burst #1.
In an exemplary OFDM/OFDMA based communication system, a subchannel is a logical channel composed of multiple subcarriers, and the number of subcarriers constituting a subchannel depends on the system configuration. That is, the IEEE 802.16e OFDMA system allocates the resources in a form of a subchannel composed of a group of subcarriers in consideration of network status, to the subscriber stations.
In the meantime, the base station performs scheduling to efficiently assign the resources so as to maximize the network throughput and minimize the transmission delays of individual subscriber stations. For achieving these objectives, a scheduling algorithm should be designed in consideration of various factors such as the inter-sector interference, UL-MAP/DL-MAP overheads, expansion of cell coverage area, energy conservation of the subscriber stations, and stability of the links between the base station and subscriber stations, etc.
In the frame structure of FIG. 2, the DL-MAP contains DL-MAP Information Elements (IEs) to describe DL bursts corresponding to the subscriber stations. Accordingly, as the number of the subscriber stations increases, the size of the DL-MAP increases. Also, the increment of the number of the UL-MAP IEs causes the increase of the UL-MAP size. A large DL-MAP and UL-MAP results in a large amount of overhead by reducing the burst regions of the DL and UL subframes of each frame.
Since the DL and UL subframes are fixed in size, the increments of the DL-MAP and UL-MAP diminish the DL burst region of the frame, resulting in a reduction of a data rate per subscriber station and a reduction in a throughput of the entire network.